This invention relates to position sensors or encoders such as an optical shaft angle encoder which produces electrical signals indicative of the angular position of the shaft. In particular, it can improve accuracy by balancing leakage currents to photodiodes which detect changes in position.
Incremental optical motion encoders are used to resolve the position and movement of an object along a particular route. Such encoders generally include a light source for emitting a light beam, light modulation means for modulating the light beam in response to movement of the object along the route and a detector assembly for receiving the modulated light and producing electrical signals indicating the amount of light received by the detectors. As the light is modulated in response to the movement of the object, each electrical signal from the detector assembly produces a wave form. The position of the object along its route determines the position of each signal on its particular wave form; that is, the phase of each signal. Thus, the electrical signals from the detectors can be used to indicate the change in location of the object along the route. Two or more properly out-of-phase signals from separate detectors can be used to indicate both change in location and direction of movement.
For an incremental motion encoder to produce an indication of the absolute position or location of the object along its route, an index pulse is generated at least once along the route. The incremental signals can be used to count incremental movement from the index pulse and if the position of the object is known at the index pulse, the absolute position of the object at any place along the route can be determined. Thus, to provide an indication of absolute position, change in location, and direction of movement, an incremental encoder generally requires three channels of information. Two channels are derived from two or more out-of-phase encoder signals that are produced throughout the route of the object and the third is an index signal that is produced at least once along the route and at a known position of the object.
In an exemplary embodiment, such a position encoder or movement detector may be used for measuring the angular position of a shaft. Depending on the use of such a shaft angle encoder, a high degree of resolution and accuracy may be needed. It is not unusual to specify a resolution of 2,000 increments per revolution of the shaft. Accuracy of the correlation between the signal from the encoder and the actual mechanical position of the shaft or other object is also of importance. Mechanical alignment discrepancies in assembling apparatus can adversely affect accuracy. It has also been found that electrical effects can degrade accuracy.
An optical encoder useful for an understanding of this invention is described in U.S. Pat. No. 4,691,101, by Leonard, the subject matter of which is hereby incorporated by reference. FIGS. 1 to 4 are from that patent. FIGS. 1 to 3 are common to practice of this invention as much as practice of the invention in the Leonard patent. FIG. 4 is representative of the prior art as represented by the Leonard patent.
In an embodiment of this invention as illustrated in FIGS. 1 to 3, an encoder module 1 provides a collimated light beam and has light detectors 7 to receive the light beam after it has been modulated by a code wheel 3. A light emitting diode 9 provides light having a wavelength of approximately 700 nanometers, however, any frequency of electromagnetic radiation having a wavelength substantially shorter than the relevant dimensions of the encoder may be utilized. An emitter lens 11 is positioned to receive the light from the LED 9 and provide a collimated beam of light.
The code wheel 3 is concentrically mounted on a shaft 5 to rotate with the shaft and modulate the light beam with its optical track 17. The optical track has alternating transmissive sections 13 and nontransmissive sections 15 of equal width. One transmissive section and one nontransmissive section make up one pitch of the code wheel. As the wheel rotates, the alternating sections permit light from the LED to pass or not pass, thereby illuminating or not illuminating the photodetectors 7.
In an exemplary embodiment the code wheel has 500 transmissive sections and an equal number of nontransmissive sections. These sections have a trapezoidal shape since they are located immediately adjacent to one another on a circular track. An exemplary nominal width of each transmissive section is 62 microns, and the radial length of each section is 750 microns. The code wheel is made of an optically opaque material, such as stainless steel, and has a diameter of approximately 22 millimeters. The transmissive sections may be holes masked and etched through the disk.
In one embodiment illustrated in the Leonard patent and reproduced in FIG. 4, there are several groups of four light detectors 7a to 7d. A group of four light detectors has approximately the same size and shape of one transmissive section 13 and one nontransmissive section 15 on the code wheel. Individual light detectors 7a to 7d have a trapezoidal shape with an exemplary maximum width of 33 microns, a minimum width of approximately 29 microns with a gap of about 8 microns between individual light detectors.
The light detectors are photodiodes fabricated on a semiconductor chip using standard bipolar semiconductor technology. As illustrated in FIG. 4, a group of light detectors are placed in a one dimensional array as close to one another as the bipolar semiconductor technology will allow. Dummy photodiodes 10 and 12 are located on each end of the array to minimize the effect of stray light on the functioning light detectors 7a to 7d. In a shaft angle encoder, the photodiodes are in an arc having the same radius as the track 17 on the code wheel.
With such an arrangement of four light detectors, four output signals are produced having the same shape but offset from one another by multiples of 90 degrees (electrical degrees). These four signals in quadrature are compared for determining the amount and direction of rotation of the code wheel.
In an embodiment as mentioned in the Leonard patent there are seven groups of four light detectors. The signals from the "a" detectors in all the groups are combined. Similarly, all of the signals from the seven "b" detectors are combined, etc. By having plural groups, a larger detector area is illuminated for greater signal strength while still maintaining high resolution. Regardless of the number of groups, there are four signals and two channels of information. Commercial embodiments have been built with 12 groups of four detectors each. In such an embodiment, 48 active detectors are arrayed in an arc with one dummy photodetector at each end.
As described in the Leonard patent, one channel of output information is obtained by summing the outputs of the "a" and "b" detectors, and comparing that with the sum of the outputs of the "c" and "d" detectors. The other channel of information is obtained by comparing the sum of the "b" and "c" detectors with the sum of the "a" plus "d" detectors. The relevant logic equations are a+b&gt;c+d and a+d&gt;b+c.
A source of error in accuracy of the correlation between the actual position of the code wheel and the electrical signals, is a so-called duty cycle error. The logical output of each channel of information should be one state (for example, ON) for 180 degrees (electrical) and the other state (OFF) for 180 degrees. This permits accurate location of the beginning and end of each transition between illuminated and not illuminated. A duty cycle error occurs when the two states are not each 180 degrees but are appreciably different from each other. Under those circumstances, the actual position of the code wheel cannot be known with the same accuracy.
There are times when it is desirable to reduce the number of groups of photodetectors for manufacturing, processing, and testing considerations. In such embodiments it is sometimes found that duty cycle errors of as much as 35 electrical degrees, may occur. It is desirable to provide means for reducing such duty cycle errors. It is also desirable to provide such a technique in a single step in the process for fabricating the semiconductor chips on which the photodetectors are formed.